Thread: what is the range of rand()

  1. #1
    Registered User
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    Jan 2011

    what is the range of rand()

    Does rand() not generate numbers between 0 to 32,767.

    HOwever, when I run the below code, I'm getting huge numbers like:

    717154347, 81702301

    #include <stdbool.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <time.h>
    int main()
      srand((unsigned) time(NULL));
      printf("number is: %d", rand());
      return 0;

    example output
    Last edited by bos1234; 06-03-2012 at 07:45 AM.

  2. #2
    Registered User
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    Dec 2011
    It produces numbers in range of its return type, in this case, int as prototyped.

    The int type is defined by the standard to be at least two bytes, which gives the signed range of a short, or -32768 through 32768. However, on most modern computers, it's four bytes, giving an exponentially larger range.

  3. #3
    and the hat of int overfl Salem's Avatar
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    Aug 2001
    The edge of the known universe
    stdlib.h should have something like
    #define RAND_MAX 2147483647

    Every ANSI/ISO system should have this, and it should be at least 32767
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  4. #4
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    Jun 2005
    If you want values between 0 and 32767, one technique is to compute rand()%32767 to obtain the lower order bits of the value returned by rand(). Another possible approach is to compute rand()/(RAND_MAX/32767 + 1) - which extracts the higher order bits of the result returned by rand(). There are other possible approaches as well.

    Note, however, that such approaches can destroy the randomness of the resultant values - depending on how rand() is actually implemented. If you care about how random the values actually are, then test to be sure.
    Right 98% of the time, and don't care about the other 3%.

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